This invention relates to a resin composition suitable for marking with a laser and a method for laser marking.
The laser beam provides a means of writing, bar coding and decorative marking of plastics. This technique is advantageous over current printing technologies because of the ease at which the layout can be adjusted using graphic computer programs and also integrated into the production line. Laser marking enables a contact-free procedure even on soft, irregular surfaces that are not readily accessible. In addition it is ink-free which makes it long-lasting and solvent-free and, thus, more friendly to the environment. Speeds up to 10,000 mm/sec are possible with a CO2 laser while Nd:YAG laser allows up to 5000 mm/sec.
There are several laser types available for marking plastic surfaces. The Excimer laser with the frequency in the range of 196-351 nm leads to the marking of plastic surfaces by photochemical ablation or reaction. Using Nd:YAG laser at lower power levels at 532 nm provides laser marking by leaching or selective bleaching of dyes and pigments while the Nd:YAG laser at 1064 nm leads to laser marking by carbonization, sublimation, discoloration, thermochemical reaction, foaming and engraving. The CO2 laser at 10600 nm enables laser marking by thermochemical reaction, melting, vaporizing and engraving.
In many instances, it is desirable to form a dark contrast on a light background. EP 0 111 357 uses metal silicates to obtain black markings on articles having a polyolefin surface. U.S. Pat. No. 4,578,329 to Holsappel describes the use of a silicon compound, preferably a metal silicate, e.g. calcium-metasilicate or kaoline to give a black mark in the laser struck areas of a polyolefin.
U.S. Pat. No. 5,489,639 to Faber et al describes the use of copper phosphate, copper sulfate and copper thiocyanate with a thermoplastic resin to give dark markings. EP 400,305 describes copper hydroxy phosphate and EP 697,433 describes the use of copper sulfate. JP 04052190 to DAINIPPON INKandCHEM KK describes a laser marking method giving high contrast black images by laser irradiating surface of resin composition containing bismuth, nickel and/or copper. Mentioned is the use of copper oxalate and copper citrate components that are known to cause splay and/or discoloration at the processing temperatures typically used for engineering thermoplastics like PBT, PP and PA.
It is desirable to make further improvements in laser marking materials of the polyester type. In particular, a desired color combination is a light background color and a dark contrast color in the laser treated areas. In particular, it is desirable to obtain a dark contrast color in the laser treated areas using a Nd:YAG laser. With increased power output/writing speed Nd:YAG lasers are nowadays more and more preferred, based on their flexibility in terms of text and images. The Nd:YAG laser enables laser marking based on several phenomena, such as melting, thermochemical reaction, vaporizing and carbonization.
The present invention is directed to provide crystalline resin compositions containing ingredients selected to enhance the laser marking of resins with the laser so light background coloration can be achieved with distinct and secure dark colored markings in the laser treated areas. The ever increasing demand for higher laser marking speeds and productivity combined with good contrast between the laser-marked part and the background stretches today""s additive technology. In fact with today""s technology the new targets are hard if not impossible to reach.
For copper salts such as copper hydroxy phosphate (EP 400 305), copper phosphate and copper sulfate (EP 697 433), a possible mechanism is the conversion of the copper salt are converted to copper oxide, yielding a black marking. Organic copper salts, like copper carbonate, copper oxalate are even more effective, probably because the conversion to copper oxide occurs at lower temperatures. However, these materials cannot effectively be used in relatively high-melting thermoplastics like PBT, PET, PP and the like because of discoloration during compounding or molding or issues related to the formation of volatile by-products.
It was surprisingly found that the copper fumarates and copper maleates did not show this kind of splay or degradation and yielded very black markings. It outperforms copper pyrophosphates and copper phosphates, particularly at low loadings. Moreover, these copper fumarates and copper maleates comply with the environmental labels like Blue Angel. Processing studies in PBT show that the copper fumarates can be compounded at melttemperatures up to 300xc2x0 C. without any problem with splay or degradation.
A resin composition having laser marking properties comprises a polycrystalline thermoplastic resin such as a polyester or polyamide, a sufficient amount of light pigment for forming a light background coloration, and an effective amount of marking to form dark colored markings in laser struck areas. The marking agent is selected from the group consisting of copper fumarates and copper maleates and mixtures thereof.